润湿性影响薄液膜沸腾传热的分子动力学研究

纳米尺度下表面润湿性如何影响薄液膜沸腾传热仍是一个具有挑战性的研究.本研究采用分子动力学方法探讨壁面润湿性影响纳米尺度薄液膜沸腾换热的机制.结果发现:亲水表面能够显著地提升沸腾换热性能,有较早的沸腾起始时间,较高的升温速率、热流密度和界面导热率,以及较小的界面热阻.通过建立二维表面势能模型,揭示表面润湿性影响纳米尺度沸腾换热的机理.亲水壁面的表面势能为-0.34 eV,而疏水壁面的表面势能仅为-0.09 eV,提升表面润湿性强化沸腾传热的本质原因是表面势能绝对值的提高.此外,通过计算分子间的相互作用能,揭示了纳米尺度下亲疏水壁面的成核机理.水分子-亲水壁面、水分子-疏水壁面和水分子内部的相互作用能分别为1.57,0.26和0.48 eV/nm².亲水表面的界面能大于水分子内部的相互作用能,因此亲水表面上气泡成核发生在水膜内部;疏水表面上的界面能比水分子内部的相互作用能弱,疏水表面的气泡成核发生在固/液界面处.本研究揭示了表面润湿性如何影响纳米尺度薄液膜沸腾传热和气泡成核的主要机制.     

纳米结构影响沸腾换热特性的分子动力学模拟

采用分子动力学方法模拟液体在纳米结构表面的快速沸腾过程.主要研究了纳米结构表面粗糙度以及栏栅形和棋盘形两种排列方式对液体快速沸腾过程以及换热特性的影响.研究结果表明,随着纳米结构表面粗糙度的增加,栏栅形和棋盘形纳米结构表面液体沸腾起始时间均提前.当栏栅形和棋盘形纳米结构表面粗糙度相同时,棋盘形纳米结构表面会进一步缩短液体沸腾起始时间.形成这种现象的原因是纳米结构表面粗糙度的增加,增加了固液接触面积,提高了初始时刻热通量,减小了固液界面热阻,导致表面附近液体动能增大,增大了液体高度方向的温度梯度,有利于液体发生沸腾.当纳米结构表面粗糙度相同时,棋盘形纳米结构表面具有较小的界面热阻,从而缩短了沸腾所需要的时间.     

纳米颗粒影响沸腾换热特性的分子动力学模拟

本文采用分子动力学方法对纳米流体在金属表面的沸腾现象进行研究,揭示纳米流体的沸腾换热机制。模拟系统分为蒸汽氩、液体氩、固体铜板和铜纳米颗粒四部分。着重探讨了近壁区的纳米颗粒对沸腾换热的影响。在高过热度和低过热度两种加热情况下,探讨纳米流体和基础流体的沸腾现象。结果表明纳米颗粒的添加对沸腾过程有显著的影响:氩的温度、系统净蒸发率显著提高。近壁区的纳米颗粒做无规则运动并且不随流体运动。颗粒的尺寸及过热度对强化效果有显著影响。     

新型共沸混合工质HC170／FC116沸腾传热实验研究

实验测量了新型共沸混合工质HC170/FC116的池核沸腾传热特性.实验测量的加热面为紫铜表面,热流密度范围为50 kW/m2～300 kW/m2.同时实验结果与复叠温区常用制冷剂R503和R508B的传热性能进行了比较,发现新型共沸混合工质HC170/FC116的传热性能高于R508B.最后对实验数据回归得到了共沸混合工质传热系数计算关联式,此关联式的计算值与实验数据的偏差在士10%以内.     

有机分子超薄膜的结构对摩擦的影响

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH}和C₁₇H₃₁COOH (n=12,13,14,15,16,17)组成的混合单层Langmuir-Blodgett(LB)膜间的摩擦特性, 探究了膜结构的变化对超薄膜的摩擦的影响. 结果显示. 在滑动过程中, 随着n的增加, 膜内分子的运动受到邻近分子的约束逐渐增加, 膜结构的稳定性也逐渐增加, 其剪切压逐渐减小, n=17时的剪切压最小. 在两单层膜之间无氢键形成; 而混合膜内的分子之间形成的氢键是单层膜结构稳定的主要因素, 其中n=16时形成的氢键最稳定, 但全部由相同C₁₇H₃₁COOH分子组成的单层膜的滑动效果最好. 分子的弯曲形变能对剪切压影响非常小.     

结构高度对纳米液滴碰撞壁面过程的影响

液滴与壁面的碰撞过程广泛存在于自然界以及各类生产生活中,探究其传热传质作用机理以及形态演变对发展表面自清洁、喷墨打印、强化滴状冷凝、抗结冰结霜等技术都具有重要意义。为了探究结构高度在碰撞过程中的作用,本文通过分子动力学模拟对纳米液滴与壁面的碰撞过程进行了研究。构建了具有方柱状结构的粗糙表面模型,研究了壁面上纳米柱状结构的高度对碰撞过程的影响,记录了液滴的变形过程,并对相关的动力学特征以及能量变化进行了分析。     

混合纳米结构增强水蒸发的分子机制

本文通过分子动力学模拟研究了亲疏水混合纳米结构表面对水蒸发的分子尺度影响机制。分子动力学模拟结果表明混合润湿性纳米结构表面可有效提升水的蒸发速率。通过分析水分子与纳米结构表面之间的相互作用,发现混合纳米结构表面可以调节水分子的蒸发屏障。本文有助于理解纳米结构表面水蒸发的物理机制,为设计高效的界面蒸发结构提供有效指导。     

载气汽油在水平螺旋扁管管束中的沸腾换热

以汽油-空气为介质,在不凝性气体质量含量不超过5%时,对导程为200mm的螺旋扁管管束中的沸腾换热进行了实验研究,分析了沸腾换热系数随两相质量流量的变化规律以及流动压降随空气流量的变化规律。得到了相应条件下,载气汽油在该导程的螺旋扁管管束中的沸腾换热实验关联式。     

Molecular dynamics study of the primary ferrofluid aggregate formation

Investigations of the phase transitions and self-organization in the magnetic aggregates are of the fundamental and applied interest. The long-range ordering structures described in the Tománek's systematization (M. Yoon, and D. Tománek, 2010 [1]) are not yet obtained in the direct molecular dynamics simulations. The resulted structures usually are the linear chains or circles, or, else, amorphous (liquid) formations. In the present work, it was shown, that the thermodynamically equilibrium primary ferrofluid aggregate has either the long-range ordered or liquid phase. Due to the unknown steric layer force and other model idealizations, the clear experimental verification of the real equilibrium phase is still required. The predicted long-range ordered (crystallized) phase produces the faceting shape of the primary ferrofluid aggregate, which can be recognized experimentally. The medical (antiviral) application of the crystallized aggregates has been suggested. Dynamic formation of all observed ferrofluid nanostructures conforms to the Tománek's systematization.     

Investigation of structural and electronic properties of β- HgS: Molecular dynamics simulations

The pressure induced phase transition of β-HgS is studied using an ab initio molecular dynamics simulation. The structural phase transformation from the zinc-blende structure to the NaCl-type structure (space group $Fm\overline{3}m$) and from this structure to CsCl-type structure ($Pm\overline{3}m$) with the application of hydrostatic pressure is predicted. Additionally, the electronic properties of HgS and various physical properties such as the lattice constants, the bulk modulus and the pressure derivative of the bulk modulus are revealed. Furthermore, these phase transitions are obtained using the total energy and enthalpy calculations. According to these calculations these transformations are occurring at about 20?GPa and 28?GPa for $F\overline{4}3m$ →$Fm\overline{3}m$ and $Fm\overline{3}m$ →$Pm\overline{3}m$, respectively.     

The study of hydrophobicity in water–methanol and water–tert-butanol mixtures

Molecular dynamics simulations have been performed for water–methanol and water–tertiary butyl alcohol mixtures in the water rich region. Nonlinear heat capacity and viscosity behavior as functions of concentration has been examined at the molecular level. The collective modes leading to enhancement of potential energy fluctuation and as consequence to heat capacity maximum has been found and discussed.     

闪锌矿相GaN热容和格林爱森参数

利用壳层模型分子动力学方法,在高温高压条件下对闪锌矿相GaN的压力体积关系、定压和定容热容及格林爱森参数进行了比较研究,其中Ga-Ga、Ga-N和N-N三组离子对间的相互作用通过极化势模型来描述,即出于对半导体材料GaN离子特性的考虑而分别给予Ga离子和N离子以两组不同大小的电荷．结果表明计算得到的环境条件下的热力学参数和最近的其他理论结果吻合,同第一性原理计算结果比较,定容热容在低温下的差别可以用来解释不同方法所采用的不同近似机制．最后在300?2000 K和0?40 GPa条件下,对闪锌矿相GaN的物理特性做了总结．     

微槽道中纳米流体沸腾换热特性研究

为研究纳米流体在微槽道中的沸腾换热特性及规律,分别以去离子水和体积浓度为0.2%、0.5%的水基Al2O3纳米流体为工质进行试验,研究质量流速、热流密度、进口过冷度、槽道尺寸等因素对沸腾传热系数的影响及其两相摩擦压降与出口干度的关联分析和沸腾换热关联式对比拟合.试验结果表明:在一定热流密度和质量流速下,传热系数随槽道尺...     

Quantification of the water boiling heat transfer in micro-structures by image analysis

The heat transfer performance of a micro-vaporizer has been measured by conventional methods (using temperatures, flow rates, effective power input). The study was carried out for laminar flow in channels (5 mm×3 cm×200 μm) micro-structured with square obstacles to increase the specific area. The results show that high heat transfer coefficients (1300– 2500 W m⁻²/C⁻¹) can be reached in such a micro-structured channel. Image analysis was done to estimate the volume vapour fraction, which can be converted into the mass vapour fraction using a slip ratio and avoids the need for any temperature or electric power input measurements. The estimation of this slip ratio is discussed in this paper.     

大空间和毛细管内液氮池沸腾传热的实验研究

文中以直径50μm,长20mm的磷青铜丝作为加热丝和测温元件,采用控制热流密度的方式测量了0°,30°,60°和90°倾角下大空间和玻璃毛细管内液氮的沸腾曲线,分析了毛细管对核态沸腾传热的影响以及管径和倾角对临界热流密度的综合影响。结果表明,在实验管径内,毛细管对于核态沸腾传热有明显的强化作用;并存在一最佳管径,可在30°~90°倾角范围内获得最大的CHF值,并且其值高于大空间时的CHF。     

Ultrahigh heat transfer enhancement using nano-porous layer

Heat transfer enhancement is one of the key issues of saving energies and compact designs for mechanical and chemical devices and plants. We discover an ultrahigh convective heat transfer performance compared to the well-known heat transfer correlations caused by a nano-particle porous surface: the maximum increase of heat transfer coefficient was around 180%. This nano-particle porous layer can be formed on the substrate surface by an etching with some acids or alkalis including around 100 nm size nano-particles made from copper oxide, carbon nano-tube and aluminium oxide. Moreover, we have done some experiments using a co-current flow heat exchanger consisting of hot and cold water-channels and obtained an ultrahigh heat transfer performance: over 200% increase compared to the conventional correlation. On the other hand, in order to theoretically investigate effects of nano-particle porous layer structures on the surface energy transfer, the energy transfer from fluid to the heat transfer surface is calculated by a classical molecular dynamics method. Energy transfer to the surface from the fluid strongly depends on the surface structures in nanometre scale that affect the static structure and dynamic behaviours of fluid molecules in the vicinity of the surface.